They're obviously not going to be in-depth scholarly treatises (they come right out and say they're brief), but are they at least factually accurate even if they're condensed and simplified?
I prefer the "Very Short Introduction" series by oxford. They are all written well, using good source analysis and objectivity. They contain little to no fluff, giving the facts and that is about it. They cover a huge variety of topics,(not just history) I would recommend them to anybody looking for a short read on a topic.